Biochemical and in Vivo Characterization of a Small, Membrane

Molecular Imaging Center, Mallinckrodt Institute of Radiology, Department of ... Molecular Biology & Pharmacology, Washington UniVersity Medical Schoo...
0 downloads 0 Views 671KB Size
Download PDF
Biochemistry 2007, 46, 4055-4065

4055

Biochemical and in ViVo Characterization of a Small, Membrane-Permeant, Caspase-Activatable Far-Red Fluorescent Peptide for Imaging Apoptosis† Kristin E. Bullok,‡ Dustin Maxwell,‡ Aparna H. Kesarwala,‡ Seth Gammon,‡ Julie L. Prior,‡ Margaret Snow,§ Sam Stanley,§ and David Piwnica-Worms*,‡,| Molecular Imaging Center, Mallinckrodt Institute of Radiology, Department of Molecular Microbiology, and Department of Molecular Biology & Pharmacology, Washington UniVersity Medical School, St. Louis, Missouri 63110 ReceiVed September 20, 2006; ReVised Manuscript ReceiVed January 12, 2007

ABSTRACT: Apoptosis is an important process involved in diverse developmental pathways, homeostasis, and response to therapy for a variety of diseases. Thus, noninvasive methods to study regulation and to monitor cell death in cells and whole animals are desired. To specifically detect apoptosis in ViVo, a novel cell-permeable activatable caspase substrate, TcapQ647, was synthesized and Km, kcat, and Ki values were biochemically characterized. Specific cleavage of TcapQ647 by effector caspases was demonstrated using a panel of purified recombinant enzyme assays. Of note, caspase 3 was shown to cleave TcapQ647 with a kcat 7-fold greater than caspase 7 and 16-fold greater than caspase 6. No evidence of TcapQ647 cleavage by initiator caspases was observed. In KB 3-1 or Jurkat cells treated with cytotoxic agents or C6-ceramide, TcapQ647 detected apoptosis in individual- and population-based fluorescent cell assays in an effector caspase inhibitor-specific manner. Further, only background fluorescence was observed in cells incubated with dTcapQ647, a noncleavable all D-amino acid control peptide. Finally, in ViVo experiments demonstrated the utility of TcapQ647 to detect parasite-induced apoptosis in human colon xenograft and liver abscess mouse models. Thus, TcapQ647 represents a sensitive, effector caspase-specific far-red “smart” probe to noninvasively monitor apoptosis in ViVo.

Apoptosis is an organized, programmed cell death process by which cells are eliminated from tissues without eliciting an immune response (1). Biochemically, the process involves cellular detection of a death signal; protease activation for degrading organelles, critical proteins, and DNA; and the packaging of degradation products into apoptotic bodies, vesicles that are engulfed by macrophages (2). Visually, apoptosis leads to characteristic morphological features such as chromatin condensation, membrane ruffling, and blebbing (2, 3). Apoptosis is initiated during developmental processes, within various diseases, and upon effective chemo- and radiation therapy (4). Because of this, noninvasive methods to monitor the progression of apoptosis in ViVo are desired. Successes in imaging apoptosis-related therapeutic efficacies in ViVo have been achieved with scintigraphy and fluorescence imaging, using molecular imaging probes composed of radionuclide- or optically-labeled annexin V, as well as with MRI (5-9). Annexin V is a protein that binds phophatidylserine (PS), a phospholipid that is externalized during apoptosis. Detection of externalized PS usually correlates to apoptosis; however, false positives can occur † This work was supported by NIH Grants P50 CA94056, RO1 CA82841, and RO1 AI30084. * Correspondence to this author at Mallinckrodt Institute of Radiology, Washington University Medical School, 510 S. Kingshighway Blvd., Box 8225, St. Louis, MO 63110. Tel: 314-362-9356. Fax: 314362-0152. E-mail: [email protected]. ‡ Molecular Imaging Center, Mallinckrodt Institute of Radiology. § Department of Molecular Microbiology. | Department of Molecular Biology & Pharmacology.

where annexin V diffuses into necrotic, membrane-leaky cells and binds to internal PS. Another potential disadvantage of an annexin V probe is its size (36 kDa), resulting in a tendency for limited diffusion into the zones of high turgor pressure characteristic of tumor tissues (10). A similar approach for detecting cell death using a biotinylated C2domain of synaptotagmin, another PS-binding protein (11), has been demonstrated by labeling the domain with streptavidin-fluorescein (11). However, the disadvantages are similar to those of annexin V. While these techniques have provided images corresponding to cell death, such probes bind in a one-to-one ratio that potentially leads to limited signal generation. One method to improve signal generation would be use of an imaging probe employing an amplification strategy directed toward apoptosis-specific enzymes, such as the caspase (cysteinyl aspartate-specific protease) family. Currently, 13 caspases have been identified and categorized into two main groups: initiator and effector caspases (1, 3). Expressed as heterodimeric zymogens, caspases are activated by proteolytic maturation or allosteric interactions (3, 12). Upon activation, caspases exclusively cleave substrates C-terminally to an aspartic acid residue. As their name implies, initiator caspases receive and transmit proapoptotic signals to effector caspases. Effector caspases then progressively degrade the cell leading to eventual cell death. Thus, activation of effector caspases generally indicates cellular commitment to apoptosis. Initiator and effector caspases are distinguished by their substrate preferences with initiator caspases (caspases 2, 8, 9, and 10) preferentially recognizing

10.1021/bi061959n CCC: $37.00 © 2007 American Chemical Society Published on Web 03/10/2007

4056 Biochemistry, Vol. 46, No. 13, 2007

Bullok et al. Previously, we presented the chemical synthesis and initial characterization of this cell permeable caspase substrate, TcapQ647 (18). We now present detailed biochemical, cellular, and in ViVo analysis of TcapQ647 to demonstrate selective cleavage by effector caspases and promising applications in optical imaging of apoptosis in ViVo. METHODS

FIGURE 1: Schematic of TcapQ647 activation following cleavage by effector caspases.

the W/HXXD sequence over the effector caspase (caspases 3, 6, and 7) recognition sequence, DXXD (13, 14). These short recognition sequences have been synthesized as substrates and inhibitors to better understand caspase activity in Vitro and in cellulo. Recently, the resulting tetrameric peptides have been utilized for therapeutic (4) and imaging purposes (15-17). More specifically, a variety of activatable fluorescence substrates are commercially available to detect effector caspase activity. However, a majority of these substrates emit fluorescence at wavelengths that are highly absorbed by tissue ( caspase 7 > caspase 6 as determined from the apparent kcat values (Table 1). Upon establishing the kinetics of TcapQ647 cleavage by effector caspases, we determined the utility of TcapQ647 to detect the presence of apoptosis in individual living cells using confocal microscopy. For these studies, adherent KB 3-1 cells were grown on 35 mm glass-bottom microwell dishes. Cells remained untreated or treated with doxorubicin (5 µM), a drug which has been shown to induce apoptosis via caspase activation (31). After treatment, cells were

4060 Biochemistry, Vol. 46, No. 13, 2007

Bullok et al.

FIGURE 4: Intracellular activation of TcapQ647 during apoptosis. (A) Untreated KB 3-1 cells incubated with TcapQ647, bis (L-aspartate) rhodamine 110 (D2R), and PI. (B) Doxorubicin-treated (5 µM) KB 3-1 cells incubated with TcapQ647, D2R, and PI. (C) Doxorubicin-treated KB 3-1 cells incubated with noncleavable dTcapQ647, D2R, and PI. (D) Doxorubicin-treated HeLa cells incubated with TcapQ647, D2R, and PI. (E) Doxorubicin-treated MCF-7 cells incubated with TcapQ647, D2R, and PI. Red fluorescence indicates activation of TcapQ647, green fluorescence indicates activation of D2R, yellow fluorescence indicates colocalization of D2R with TcapQ647, and purple indicates colocalization of PI with TcapQ647.

simultaneously incubated with TcapQ647 (4 µM) and a commercial quenched fluorescence caspase substrate, bisaspartate rhodamine (D2R; 5 µM). For control experiments, cells were incubated with the nonhydrolyzable all D-amino acid peptide, dTcapQ647, along with D2R. For all conditions, propidium iodide was added to the cells prior to confocal microscopy analysis. D2R activation was observed as green fluorescence, while TcapQ647 activation was observed as red fluorescence. PI staining of membrane-compromised cells was falsely colored as blue fluorescence for ease of overlay analysis. As can be seen, TcapQ647 activation was observed in doxorubicin treated cells but not in untreated cells (Figure 4). Colocalization of D2R and TcapQ647 activation was also observed (Figure 4B; yellow cells). As compared to TcapQ647, a larger number of cells demonstrated D2R activation, consistent with its utility as a general caspase substrate, while TcapQ647 is a specific effector caspase substrate. Cells positive for both TcapQ647 activation and PI staining were also observed (Figure 4B, purple cells). Control studies with noncleavable dTcapQ647 resulted in low fluorescence background, indicating that the observed fluorescence in cells incubated with TcapQ647 was due to enzymatic proteolysis rather than conformational change of the quenched peptide (Figure 4C). Previous inhibition studies (18) support these results and demonstrated that TcapQ647 activation was inhibited upon treatment of cells with the effector caspase inhibitor, D(OMe)QMD(OMe)-FMK. TcapQ647 activation was further demonstrated in two other cell lines (HeLa, MCF-

7) treated with doxorubicin (Figure 4D,E), indicating the general applicability of the imaging probe. We next determined the utility of TcapQ647 to detect increasing apoptosis in a cell population following treatment of cells with increasing concentrations of drug. For this concentration-response assay, Jurkat cells were utilized as they are a well-established apoptotic cell model and are grown in suspension. Induction of apoptosis by C6-ceramide has been well characterized in Jurkat cells (32, 33). Thus, apoptosis was induced in the Jurkat cell population by incubation with increasing concentrations of C6-ceramide as described (Methods). In Jurkat cells, the rate of TcapQ647 activation increased proportionately with ceramide concentrations (Figure 5A). These observations correlated with decreases in cell viability as measured by MTS assay (Figure 5B). Treatment with greater than 30 µM C6-ceramide resulted in substantial decreases in cell viability as observed both by endpoint MTS analysis (Figure 5B) and phase contrast microscopy (data not shown). No increase in fluorescence was observed when Jurkat cells were incubated with the noncleavable control peptide, dTcapQ647 (data not shown). Finally, TcapQ647 activation was completely abrogated in cells pretreated with the effector caspase inhibitor D(OMe)QMD(OMe)-FMK (Figure 5C). These data indicated that the extent of TcapQ647 activation was proportional to the amount of apoptosis occurring in a given cell population.

Characterization of a Smart Imaging Peptide

Biochemistry, Vol. 46, No. 13, 2007 4061

FIGURE 6: Western blot of Jurkat cell lysates following treatment for 3 h with increasing concentrations of C6-ceramide. (A) The blot was probed with anti-procaspase 3 antibody to monitor decreases in the level of procaspase 3 following treatment with C6-ceramide. To normalize the level of procaspase 3 in each lane, the blot was stripped and reprobed with anti-β-actin antibody. (B) Ratio of procaspase 3 levels to β-actin levels as determined by Western blot analysis plotted against increasing concentrations of C6-ceramide.

FIGURE 5: Jurkat cell concentration-response assay as determined by TcapQ647 activation. Jurkat cells were treated with increasing concentrations of C6-ceramide for 3 h to induce apoptosis, and then incubated with TcapQ647, and increases in fluorescence were monitored over time. (A) Initial rates of TcapQ647 cleavage were normalized to cell viability and plotted against concentration of C6-ceramide. AU represents absorption units from MTS analysis, and FU represents fluorescence units. (B) Viability of Jurkat cells as determined by MTS analysis following treatment of cells with increasing concentration of C6-ceramide. (C) Analysis of TcapQ647 activation in C6-ceramide-treated (40 µM) Jurkat cells with or without the caspase inhibitor D(OMe)QMD(OMe)-FMK (40 µM). Bars indicate SEM.

To independently confirm activation of caspase 3 in the ceramide-treated Jurkat cells, Western blot analysis of each condition from the above assay was performed. As shown, the amount of procaspase 3 decreased with increasing concentrations of C6-ceramide (Figure 6A), indicating activation of caspase 3. Western blot analysis of procaspase 3 was normalized to total protein in each lane as determined by β-actin levels. Normalized data were plotted against C6ceramide concentration (Figure 6B) to determine the concentration-response of procaspase 3 activation. The ap-

proximate EC50 value (15 µM) of C6-ceramide-induced proteolysis of procaspase 3 correlated with the concentration of C6-ceramide where activation of TcapQ647 was first observed via analysis by fluorescence spectrophotometry (Figure 5A). To determine the utility of TcapQ647 to detect apoptosis in ViVo, two E. histolytica-infected mouse models were utilized. The first model involved a human colon xenograft mouse model where sections of human colonic tissue were grafted into the subscapular region of SCID or SCID/beige mice (25). Once established, xenografts were infected with amoeba trophozoites by direct intraluminal injection. At 4 h postinjection, damage to the host intestinal lining can be observed (25). Tissue destruction following infection with E. histolytica has been associated with apoptosis (34). Because of the surface location of the xenografts, this mouse model seemed ideal for initial examination of in ViVo activation of TcapQ647 in tissues undergoing apoptosis. The second model involved direct injection of E. histolytica into a single lobe of the liver and subsequent local abscess formation or use of saline as a control. For colon experiments in ViVo, mice with bilateral xenografts were utilized. The region around the xenografts was shaved and depilated prior to infecting the left xenograft with E. histolytica and the right xenograft with parasite-free media as a control. For colon xenograft experiments 24 h postinfection, mice were imaged over time following i.p. injection of either TcapQ647 (0.1 mg/ 1.5 mL saline), noncleavable dTcapQ647 (0.1 mg/ 1.5 mL saline), or saline (Figure 7). Colon xenografts were then removed and imaged ex ViVo, and representative samples were subsequently analyzed by

4062 Biochemistry, Vol. 46, No. 13, 2007

Bullok et al.

FIGURE 7: Far-red fluorescence images of colonic xenograft in mice in ViVo. Human colonic xenografts were mock-infected as a control or infected with E. histolytica 24 h prior to i.p. injection of TcapQ647 (n ) 6), noncleavable dTcapQ647 (n ) 4), or saline (n ) 2). (A) Representative fluorescence images of mice with bilateral xenografts 60 min postinjection of the indicated solution. The mock-infected xenograft is located in the left half of each image (mouse right side) whereas the amoeba-infected xenograft is located in the right half of each image. Fluorescence and X-ray images were obtained on a Kodak Multimodality Imaging Station 4000MM. Images are falsely colored and overlaid with corresponding X-ray images. (B) Graphical representation of the normalized fluorescence intensity of in ViVo xenograft images as determined by region of interest analysis. Assigned numerals following TcapQ or dTcapQ indicate individual mice. (C) Fluorescence images of ex ViVo xenografts comparing fluorescence intensity in mock- and amoeba-infected xenografts corresponding to several mice in (A). (D) Graphical representation of the normalized fluorescence intensity of ex ViVo xenograft images as determined by region of interest analysis of ex ViVo images. Assigned numerals following TcapQ or dTcapQ indicate individual mice. Black bars, amoeba-infected xenograft; gray bars, mock-infected xenograft. Inset: Correlation of normalized TcapQ647 fluorescence intensity for amoeba-infected xenografts and TUNEL score.

FIGURE 8: TUNEL analysis of grafts from colon xenograft mouse model comparing mock- and amoeba-infected tissues. (A) Positive control for TUNEL stain. (B) Amoeba-infected xenograft from TcapQ3. (C) Mock-infected xenograft from mouse TcapQ3. (D) Amoebainfected xenograft from TcapQ1. (E) Amoeba-infected xenograft from noncleavable dTcapQ1. (F) Amoeba-infected xenograft from dTcapQ2. All images were obtained using a 40× objective on a Zeiss Axiovert 200 laser scanning confocal microscope. Assigned numerals following TcapQ or dTcapQ indicate individual mice.

TUNEL staining (Figure 8). Overall, images ex ViVo correlated with images in ViVo (Figure 7A,C). Apoptosis by

TUNEL staining was surprisingly variable in amoebainfected xenografts, being present only in selected specimens

Characterization of a Smart Imaging Peptide

Biochemistry, Vol. 46, No. 13, 2007 4063

Table 2: Mean Fluorescence Intensity of ex ViVo Liver Lobesa condition

infected

mock

liver, TcapQ647 liver, saline

6000 500

3700 *

a Livers were infected with E. histolytica or mock-infected as a control. * indicates not assessed. Mean fluorescence intensity was determined by ROI analysis of the entire lobe using Kodak Multimodality Imaging Station 4000MM 1D imaging software.

(e.g., Figure 8B,D,E,F), but not observed, as expected, in mock-infected xenografts (e.g., Figure 8C). Region of interest (ROI) analysis of TcapQ647 signal in colon xenografts (Figure 7C) correlated with the presence of apoptosis as determined by TUNEL analysis (Figure 8B-D). For example, the amoeba-infected xenograft of mouse TcapQ3 resulted in the highest fluorescence signal (5-fold greater than the background observed in the corresponding mock-infected xenograft) and also showed the highest TUNEL signal. A similar, but less dramatic result was observed in the mouse TcapQ1 (Figure 7A-D). Analysis of mouse TcapQ2, which had been infected, but showed no detectable fluorescence signal, also showed no positive TUNEL staining in the amoeba-infected xenograft. For mice injected with the noncleavable peptide dTcapQ647, little (dTcapQ2) to no (dTcapQ1) fluorescence was observed in all amoeba-infected relative to mock-infected xenografts (Figure 7). TUNEL analysis confirmed the presence of apoptosis in the dTcapQ1 amoeba-infected xenograft, but not in the dTcapQ2 amoebainfected xenograft (Figure 8E,F). Overall, for those samples analyzed by both methods, activation of TcapQ647 in ViVo correlated with the presence of apoptosis as determined by TUNEL staining (Figure 7D, inset; rs ) 0.948, n ) 4), while background signal from dTcapQ647 did not. Similar increases in fluorescence over controls were observed with ex ViVo analysis of liver lobes using the parasitic liver mouse model (Table 2). Thus, the extent of TcapQ647 activation in ViVo appeared to be indicative of the level of apoptosis present in a given tissue. DISCUSSION This study presented full biochemical and in ViVo characterization of a permeation peptide-based imaging substrate specific for effector caspases. These kinetic studies indicate that TcapQ647 is strongly and preferentially cleaved by effector caspases over initiator caspases. Furthermore, in contrast to our pilot data suggesting similar kinetics for caspases 3 and 7 (18), full analysis for TcapQ647 showed kcat values of 89 000, 13 000, and 5700 FU min-1 µM-1 for caspase 3, caspase 7, and caspase 6, respectively, indicating preferential cleavage of TcapQ647 by caspase 3. Of note, caspase 3 cleaved TcapQ647 at an absolute rate greater than the commercial substrates MCA-DEVDAP-DNP (SubIII; data not shown) and DEVD-AMC (SubII). Similarly, caspase 7 was observed to cleave TcapQ647 at a greater rate than SubII. The differences between TcapQ647 and commercial substrates are the size of the chromophore(s), the length of the peptide sequence, and the mechanism of fluorescence quenching, any of which could impact overall kinetics. Previous studies by others showed that SubII was cleaved by caspases 3 and 7 with identical Km values near 11 µM

(13, 30). The Km of TcapQ647 toward 3 and 7 were in the low µM to nM range, respectively, which would be advantageous for imaging in ViVo since substrate concentrations would likely be present within these molar ranges. Interestingly, kinetic studies herein demonstrated that caspase 3 cleaved TcapQ647 with an apparent kcat 7-fold greater than caspase 7, which is almost identical to kcat ratios for substrates reported by Thornberry et al. Thus, TcapQ647 exhibited similar biochemical properties to other probes already studied, but also provided certain affinity advantages for detecting caspases 3 and 7 in the in ViVo setting. Absorption spectrometry analysis indicated that TcapQ647 fluorescence was not quenched via a classical Fo¨rster resonance energy transfer (FRET). Instead, the absorption maximum of the TcapQ647 fluorophore, Alexa Fluor 647, was blue-shifted from 650 nm in the absence of the quencher, QSY 21, to 605 nm in the presence of the quencher, indicating the presence of a strong Coulombic interaction between the transition dipole moments of QSY 21 and Alexa Fluor 647 (35). The altered spectrum is thought to arise when such a chromophore interaction leads to the formation of an intramolecular dimer called an exciton (36) and a subsequent split in excitation energy levels (37). Depending on the electronics of the dimer, the absorption spectrum can either be blue-shifted, called an H-dimer, where radiative transitions to the lower excitation level are forbidden, or red-shifted, called a J-dimer, where radiative transitions to the higher excitation level are forbidden (35, 38, 39). Such interactions have been shown to impose secondary structure to peptide sequences positioned between chromophores (39). Taken together, these spectral and kinetic data indicate that flanking effector caspase recognition sequences with exciton-forming chromophores may impact specific conformational states, kinetics, and specificity of the peptide sequences. With respect to whole cell assays, TcapQ647 was shown to be efficiently and specifically activated by caspases in cellulo. TcapQ647 activation was demonstrated by confocal microscopy to correlate with cleavage of the general caspase substrate, D2R, on an individual KB 3-1 cell basis. In population-based fluorometric analysis, TcapQ647 activation was shown to increase proportionately with the increases in ceramide-induced Jurkat cell death. As would be expected, the observed TcapQ647 activation signals were effectively attenuated upon treatment of cells with an effector caspase inhibitor. In contrast, no increase in fluorescence intensity was observed in cells upon incubation with the all D-amino acid noncleavable peptide, dTcapQ647, on either an individual or population-based analysis. Finally, this study demonstrated the utility of TcapQ647 in detecting amoeba-induced cell death in ViVo. In mice with bilateral colon xenografts, a greater increase in fluorescence intensity was observed in E. histolytica-infected over mockinfected xenografts following TcapQ647 injection. In contrast, no significant increase in fluorescence intensity was observed following injection with the noncleavable peptide, dTcapQ647. Similar data were observed in the amoeba-infected liver lobes over mock-infected liver lobes. The extent of this TcapQ647 activation in amoeba-infected xenografts varied from animal to animal. However, TUNEL analysis of excised colonic tissue indicated that this was due to varying levels of cell death within each xenograft. Importantly, a strong positive correlation (rs ) 0.948) was observed between the amount

4064 Biochemistry, Vol. 46, No. 13, 2007 of TUNEL stain and the level of TcapQ647 activation. In contrast, little to no increase in fluorescence intensity was observed following injection with the noncleavable peptide, dTcapQ647, even in TUNEL positive tissue. In conclusion, this study demonstrated favorable properties of TcapQ647 to detect apoptosis in cultured cells and in ViVo. The incorporation of a quenched fluorescence strategy onto the cell-permeable caspase substrate resulted in little to no detectable fluorescence signal in the absence of activated caspases. By utilizing a far-red fluorophore, caspase cleavage resulted in light emission above 650 nm, where intrinsic photon absorption by cells and tissues is low. Further, the chosen quencher-fluorophore pair, QSY 21/Alexa Fluor 647, generated an enzyme substrate highly specific for effector caspases. Finally, TcapQ647 activation was shown to correlate with cellular activation of caspase 3 and with ex ViVo analysis of cell death via TUNEL staining. Modifications to the TcapQ scaffold could provide a panel of compounds with altered protease specificity and further improvements in light penetration from activation in deep tissues. ACKNOWLEDGMENT Special thanks to Vijay Sharma and Samuel Achilefu for valuable discussions, the Hope Center for Neurological Diseases at Washington University for confocal microscopy use, and the Washington University Digestive Diseases Research Core Center for tissue preparation and TUNEL staining. REFERENCES 1. Grutter, M. (2000) Caspases: key players in programmed cell death, Curr. Opin. Struct. Biol. 10, 649-655. 2. Shi, Y. (2001) A structural view of mitochondria-mediated apoptosis, Nat. Struct. Biol. 8, 394-401. 3. Kroemer, G., and Martin, S. (2005) Caspase-independent cell death, Nat. Med. 11, 725-730. 4. Fischer, U., and Schulze-Osthoff, K. (2005) New approaches and therapeutics targeting apoptosis in disease, Pharmacol. ReV. 57, 187-215. 5. Ran, S., and Thorpe, P. E. (2002) Phosphatidylserine is a marker of tumor vasculature and a potential target for cancer imaging and therapy, Int. J. Radiat. Oncol. Biol. Phys. 54, 14791484. 6. Van De Wiele, C., Lahorte, C., Vermeersch, H., Loose, D., Mervillie, K., Steinmetz, N., Vanderheyden, J., Cuvelier, C., Slegers, G., and Dierck, R. (2003) Quantitative tumor apoptosis imaging using technetium-99m HYNIC annexin V single photon emission computed tomography, J. Clin. Oncol. 21, 34833487. 7. Petrovsky, A., Schellenberger, E., Josephson, L., Weissleder, R., and Bogdanov, A. J. (2003) Near-infrared fluorescent imaging of tumor apoptosis, Cancer Res. 63, 1936-1942. 8. Schellenberger, E., Bogdanov, A. J., Petrovsky, A., Ntziachristos, V., Weissleder, R., and Josephson, L. (2003) Optical imaging of apoptosis as a biomarker of tumor response to chemotherapy, Neoplasia 5, 187-192. 9. Hamstra, D., Chenevert, T., Moffat, B., Johnson, T., Meyer, C., Mukherji, S., Quint, D., Gebarski, S., Fan, X., Tsien, C., Lawrence, T., Junck, L., Rehemtulla, A., and Ross, B. (2005) Evaluation of the functional diffusion map as an early biomarker of time-toprogression and overall survival in high-grade glioma, Proc. Natl. Acad. Sci. U.S.A. 102, 16759-16764. 10. Jain, R. (2005) Normalization of tumor vasculature: an emerging concept in antiangiogenic therapy, Science 307, 58-62. 11. Jung, H., Kettunen, M., Davletov, B., and Brindle, K. (2004) Detection of apoptosis using the C2A domain of synaptotagmin I, Bioconjugate Chem. 15, 983-987.

Bullok et al. 12. Riedl, S., and Shi, Y. (2004) Molecular mechanisms of caspase regulation during apoptosis, Nat. ReV. Mol. Cell Biol. 5, 897906. 13. Thornberry, N. A. (1997) A combinatorial approach defines specificities of members of the caspase family and granzyme B, J. Biol. Chem. 272, 17907-17912. 14. Lien, S., Pastor, R., Sutherlin, R., and Lowman, H. (2004) A substrate-phage approach for investigating caspase specificity, Protein J. 23, 413-425. 15. Smolewski, P., Bedner, E., Du, L., Hsieh, T. C., Wu, J. M., Phelps, D. J., and Darzynkiewicz, Z. (2001) Detection of caspases activation by fluorochrome-labeled inhibitors: Multiparameter analysis by laser scanning cytometry, Cytometry 44, 73-82. 16. Komoriya, A., Packard, B. Z., Brown, M. J., Wu, M. L., and Henkart, P. A. (2000) Assessment of caspase activities in intact apoptotic thymocytes using cell-permeable fluorogenic caspase substrates, J. Exp. Med. 191, 1819-28. 17. Zhang, H.-Z., Kasibhatla, S., Guastella, J., Tseng, B., Drewe, J., and Cai, S. X. (2003) N-Ac-DEVD-N′-(Polyfluorobenzoyl)R110: Novel cell-permeable fluorogenic caspase substrates for the detection of caspase activity and apoptosis, Bioconjugate Chem. 14, 458-463. 18. Bullok, K., and Piwnica-Worms, D. (2005) Synthesis and characterization of a small, membrane-permeant, caspase-activatable far-red fluorescent peptide for imaging apoptosis, J. Med. Chem. 48, 5404-5407. 19. Akiyama, S. I., Fojo, A., Hanover, J. A., Pastan, I., and Gottesman, M. M. (1985) Isolation and genetic characterization of human KB cell lines resistant to multiple drugs, Somatic Cell Mol. Genet. 11, 117-126. 20. Piwnica-Worms, D., Chiu, M., Budding, M., Kronauge, J., Kramer, R., and Croop, J. (1993) Functional imaging of multidrug-resistant P-glycoprotein with an organotechnetium complex, Cancer Res. 53, 977-984. 21. Polyakov, V., Sharma, V., Dahlheimer, J., Pica, C., Luker, G., and Piwnica-Worms, D. (2000) Novel Tat-peptide chelates for direct transduction of technetium-99m and rhenium into human cells for imaging and radiotherapy, Bioconjugate Chem. 11, 762771. 22. Luker, G. D., Flagg, T. P., Sha, Q., Luker, K. E., Pica, C. M., Nichols, C. G., and Piwnica-Worms, D. (2001) MDR1 Pglycoprotein reduces influx of substrates without affecting membrane potential, J. Biol. Chem. 276, 49053-49060. 23. Burlingham, B. T., and Widlanski, T. S. (2003) An intuitive look at the relationship of Ki and IC50: a more general use for the Dixon Plot, J. Chem. Educ. 80, 214-218. 24. Mathews, C., van Holde, K., and Ahern, K. (2000) Biochemistry, 3rd ed., Benjamin Cummings, San Francisco. 25. Zhang, Z., Wang, L., Seydel, K., Li, E., Ankri, S., Mirelman, D., and Stanley, S. (2000) Entamoeba histolytica cysteine proteinases with interleukin-1 beta converting enzyme (ICE) activty cause intestinal inflammation and tissue damage in amoebiasis, Mol. Microbiol. 37, 542-548. 26. Stanley, S. J., Becker, A., Kunz-Jenkins, C., Foster, L., and Li, E. (1990) Cloning and expression of a membrane antigen of Entamoeba histolytica possessing multiple tandem repeats, Proc. Natl. Acad. Sci. U.S.A. 87, 4976-4980. 27. Glantz, S. A. (1987) Primer of Biostatistics, 2nd ed., p 379, McGraw-Hill, Inc., New York. 28. Salgado, J., Garcia-saez, A., Malet, G., Mingarro, I., and PerezPaya, E. (2002) Peptide in apoptosis research, J. Pept. Sci. 8, 543560. 29. Ekert, P., Silke, J., and Vaux, D. (1999) Caspase inhibitors, Cell Death Differ. 6, 1081-1086. 30. Talanian, R. V., Quinlan, C., Trautz, S., Hackett, M. C., Mankovich, J. A., Banach, D., Ghayur, T., Brady, K. D., and Wong, W. W. (1997) Substrate specificities of caspase family proteases, J. Biol. Chem. 272, 9677-9682. 31. Dimanche-Boitrel, M., Meurette, O., Rebillard, A., and Lacour, S. (2005) Role of early plasma membrane events in chemotherapyinduced cell death, Drug Resist. Updates 8, 5-14. 32. Herr, I., Wilhelm, D., Bohler, T., Angel, P., and Debatin, K.-M. (1997) Activation of CD95 (APO-1/Fas) signaling by ceramide mediates cancer therapy-induced apoptosis, EMBO J. 16, 62006208. 33. Lin, C., Chen, C., Chang, W., Jan, M., Hsu, L., Wu, R., Tang, M., Chang, W., and Lin, Y. (2004) Sequential caspase-2 and

Characterization of a Smart Imaging Peptide caspase-8 activation upstream of mitochondria during ceramide and etoposide-induced apoptosis, J. Biol. Chem. 279, 4075540761. 34. Huston, C., Boettner, D., Miller-Sims, V., and Petri, W. J. (2003) Apoptotic killing and phagocytosis of host cells by the parasite Entamoeba histolytica, Infect. Immun. 71, 964-972. 35. Kasha, M. (1963) Energy transfer mechanism and molecular exciton model for molecular aggregates, Radiat. Res. 20, 5571. 36. Hernando, J., van der Schaaf, M., van Dijk, E., Sauer, M., GarciaParajo, M., and van Hulst, N. (2003) Excitonic behavior of rhodamine dimers: a single-molecule study, J. Phys. Chem. A 107, 43-52.

Biochemistry, Vol. 46, No. 13, 2007 4065 37. Bernacchi, S., and Me´ly, Y. (2001) Exciton interaction in molecular beacons: a sensitive sensor for short range modifications of the nucleic acid structure, Nucleic Acids Res. 29, e62. 38. Johansson, M., Fidder, H., Dick, D., and Cook, R. (2002) Intramolecular dimers: a new strategy to fluorescence quenching in dual-labeled oligonucleotide probes, J. Am. Chem. Soc. 124, 6950-6956. 39. Packard, B., Toptygin, D., Komoriya, A., and Brand, L. (1997) Design of profluorescent protease substrates guided by exciton theory, Methods Enzymol. 278, 15-23. BI061959N